Method and apparatus for femtosecond laser pulse measurement based on transient-grating effect

ABSTRACT

Apparatuses and method for real-time measuring ultrashort pulse shape and pulse width. Transient-grating effect on a transparent optical medium is used to generate a reference beam. A black plate with four equal-sized holes divides the incoming laser beam into four beams, one of which is attenuated and introduced an appropriate time delay relative to the other three. The four laser beams pass through a concave mirror and are focused onto a nonlinear transparent optical medium. The three beams without attenuation are used to generate a transient-grating light in the transparent medium. The transient-grating light is collinear and overlapped with the fourth attenuated beam. According to the third-order nonlinear effect, the transient-grating light has a broader spectral bandwidth and more smooth spectrum phase with respect to the incident laser. By measuring the spectral interference, the spectrum and spectral phase may be retrieved by spectral interferometry.

CROSS-REFERENCE AND RELATED APPLICATIONS

The subject application is a continuation-in-part of PCT internationalapplication PCT/CN2012/000873 filed on Jun. 25, 2012, which in turnclaims priority on Chinese patent application No. CN 201210079324.Xfiled on Mar. 22, 2012. The contents and subject matter of the PCT andChinese priority applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a femtosecond laser and method andapparatus for femtosecond laser pulse measurement based ontransient-grating effect on a transparent medium, particularly, aself-referencing spectral interferometry method and apparatus forretrieving the spectral phase and pulse width that can be used tomeasure pulse in the 200-3000 nm spectral range. The present inventionalso relates to the application of the device for measuring megahertzrepetition rate femtosecond laser pulses and single-shot femtosecondlaser pulses.

BACKGROUND OF THE INVENTION

Femtosecond laser pulses had been applied broadly in various fields suchas biological, medical, processing, communication, defense, and others.Femtosecond lasers and relative technologies have also been developedquickly. Recently, hot research fields, such as femtosecond chemistry,femtosecond nonlinear optical microscopy imaging of chemical andbiological materials, are all based on femtosecond lasers. Attosecondlaser pulse generation, X-ray laser, laboratory astrophysics, laseracceleration of electrons and protons, and other strong-field laserphysics are all taking the advantage of femtosecond laser pulses as aresearch tool. In contrast to the nanosecond and picosecond lasers,femtosecond laser processing can get much more refined and smoothsurface shape. Then it was widely used in the field of femtosecond lasermicromachining. Femtosecond laser pulses has also recently been used forophthalmic lens cutting operation, which greatly improves the qualityand safety of the kind of surgery.

In the application of the laser, the pulse shape and the pulse width ofthe femtosecond laser pulse are important optical parameters. Real-timemeasurement or monitoring of these parameters are necessary in manyexperiments. Therefore, a simple, convenient, and effective method andapparatus for laser pulse measurement and real-time monitoring isimportant and promotes the development and application of femtosecondlaser technology.

The technique for femtosecond laser pulse width measurement is evolvingas the development of femtosecond laser technology. Currently, the mostcommonly used methods include autocorrelation method, See R. Trebino,Frequency-Resolved Optical Grating: The Measurement of Ultrashort LaserPulses, Kluwer Academic Publishers (2000), frequency-resolved opticalgating (FROG) method, See R. Trebino et al., “Measuring ultrashort laserpulses in the time frequency domain using frequency-resolved opticalgating,” Rev. Sci. Instrum.68 (9), 3277-3295 (1997), and spectral phaseinterferometry for direct electric-field reconstruction (SPIDER) method,See C. Iaconis et al., “Spectral phase interferometry for directelectric-field reconstruction of ultrashort optical pulses,” Opt. Lett.23 (10), 792-794 (1998). Autocorrelation method is simple in theprinciples and structure but can not obtain the phase information offemtosecond laser pulses. FROG and SPIDER are used to get the pulsephase. However, FROG method usually need a long time to rebuild thepulse. SPIDER method usually requires a nonlinear optical crystal toconvert the generated measurement signal. Because of the phase matchingconditions in nonlinear optical crystals, each apparatus can only beadapted to a particular spectral range, thus limiting the application ofthe method in a wide spectrum range.

Recently, cross-polarized wave (XPW) generation, see A. Jullien et al.,“Spectral broadening and pulse duration reduction during cross polarizedwave generation: influence of the quadratic spectral phase,” Appl. Phys.B 87 (4), 595-601(2007), is used as a reference light forself-referenced spectral interferometry (SRSI) method, see T.Oksenhendler et al., “Self-referenced spectral interferometry,” Appl.Phys. B 99 (1), 7-12 (2010), to measure the femtosecond laser pulse. Inthis method, one incident beam is used without being divided into twobeams. In the calculation, only three simple iterative calculations areneeded to quickly obtain the spectra and spectral phase of the measuredlaser pulse, which is by far the most simple and convenient method.However, it requires the polarizer in the method. Then, the method isonly valid for a particular wavelength, which also limits theapplication of the method and apparatus within a specific spectralrange. The dispersion of the polarizer element also restricts theshortest pulse duration to be measured on 10 fs level. Recently, theself-diffraction effect based SRSI method has been used with thepolarizer and relative restriction. In the method, the beam to bemeasured is divided into three beams. The current setup of the method issomewhat complex. See J. Liu et al., “Self-referenced spectralinterferometry based on self-diffraction effect,” J. Opt. Soc. Am. B 29(I): 29-34 (2012).

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus based on thetransient-grating effect on a transparent medium by using SRSItechnique. The present invention provides a method and apparatus basedon the transient-grating effect on a transparent medium by using theSRSI technique. The method and apparatus of the present inventioneliminate the drawbacks of the conventional methods. In the method ofthe present invention, the laser to be measured is divided into fourbeams by using a black plate with four equal-sized holes. The setup issimple, easy to adjust, the data acquisition and processing is fast, andit is a one-shot measurement that can be adapted to the real-timemeasurement. The method and apparatus of the present invention is usefulfor monitoring pulses with different pulse widths and differentwavelength.

The present invention provides a method based on the transient-gratingeffect using the SRSI technique to measure the femtosecond pulse. Themethod comprises the following steps:

{circle around (1)} the transient-grating effect (the laser to bemeasured is divided into four beams here) on a transparent medium isused to generate the reference light for an SRSI measurement;

{circle around (2)} the spectral interference fringes D (ω, τ) betweenthe generated transient-grating light and the light to be measured ismeasured by a high precision spectrometer;

{circle around (3)} Based on the spectral interference fringes D (ω, τ),the spectral phase is retrieved by the SRSI calculation technique. Then,we can get the pulse width and pulse shape.

The present invention provides a first apparatus for femtosecond laserpulse measurement based on the transient-grating effect as shown in FIG.1, which is characterized in that it comprises a plate with fourequal-sized holes (as shown in FIG. 4( a)), a delay plate, a planereflective mirror, a first concave reflective mirror, a third-ordernonlinear optical medium, an iris, a second concave reflective mirror,and a spectrometer with high spectral accuracy. The relationship of thecomponents are shown as follows: the plate with the four equal-sizedholes have four holes in a square shape; the first, second, and thirdquadrants of the plane reflective mirror are coated with high reflectivefilm, and the fourth quadrant is uncoated; the femtosecond laser to bemeasured is divided into four beams after passing through the plate withfour equal-sized holes. The four beams are referred to as the first, thesecond, the third, and the fourth beams. The first, the second, and thethird beams are directly reflected by the first, the second, and thethird quadrants of the plane mirror. The fourth beam passes through thedelay plate, and then, is reflected by the uncoated fourth quadrant ofthe plane mirror. All the four beams are reflected onto the firstconcave mirror. A third-order nonlinear optical medium is located at thefocal point of the first concave mirror. The first, the second, and thethird beam are overlapped in the third-order nonlinear optical mediumand generate a transient-grating signal light. The transient-gratinglight is collinearly overlapped with the fourth beam in space. Afterpassing through the iris, the two beams are focused into thespectrometer with high spectral resolution by using the second concavereflection mirror. Thus, the interference spectrum is obtained for theSRSI measurement.

The plane mirror is a mirror of which the first, second, and thirdquadrants of the plane reflective mirror are coated with high reflectivefilm, and the fourth quadrant is uncoated as shown in FIG. 4( b).

The present invention also provides a second apparatus for femtosecondlaser pulse measurement based on the transient-grating effect as shownin FIG. 2, which is characterized in that it comprises a plate with fourequal-sized holes (as shown in FIG. 4( a)), a delay plate, a lens, aplane reflective mirror, a third-order nonlinear optical medium, aniris, a concave reflective mirror, and a spectrometer with high spectralaccuracy. The relationship of the components are shown as follows: theplate with four equal-sized holes have four holes in a square shape; thefirst, second, and third quadrants of the plane reflective mirror arecoated with high reflective film, and the fourth quadrant is uncoated;the femtosecond laser to be measured is divided into four beams afterpassing through the plate with four equal-sized holes. The four beamsare referred to as the first, the second, the third, and the fourthbeams. The first, the second, and the third beams directly pass throughthe lens and then are reflected by the first, the second, and the thirdquadrants of the plane mirror. The fourth beam passes through the delayplate and the lens, and then is reflected by the uncoated fourthquadrant of the plane mirror. A third-order nonlinear optical medium islocated at the focal plane of the lens. The first, the second, and thethird beam are overlapped in the third-order nonlinear optical mediumand generate a transient-grating signal light. The transient-gratinglight is collinearly overlapped with the fourth beam in space. Afterpassing through the iris, the two beams are focused into thespectrometer with high spectral resolution by the concave reflectionmirror. Thus, the interference spectrum is obtained for the SRSImeasurement.

The present invention further provides a third apparatus for femtosecondlaser pulse measurement based on the transient-grating effect as shownin FIG. 3, which is characterized in that it comprises a plate with fourequal-sized holes (as shown in FIG. 4( a)), a delay plate, a firstconcave reflective mirror, a third-order nonlinear optical medium, aniris, a second concave reflective mirror and a spectrometer with highspectral accuracy. The relationship of the component parts are shown asfollows: the first, second, and third quadrants of the first concavereflective mirror are coated with high reflective film, and the fourthquadrant is uncoated; the femtosecond laser to be measured is dividedinto four beams after passing through the plate with four equal-sizedholes. The four beams are referred to as the first, the second, thethird, and the fourth beams. The first, the second, and the third beamsare directly reflected by the first, the second, and the third quadrantsof the first concave reflective mirror. The fourth beam passes throughthe delay plate and then is reflected by the uncoated fourth quadrant ofthe first concave reflective mirror. A third-order nonlinear opticalmedium is located at the focal point of the first concave mirror. Thefirst, the second, and the third beams are overlapped in the third-ordernonlinear optical medium and generate a transient-grating signal light.The transient-grating light is collinearly overlapped with the fourthbeam in space. After passing through the iris, the two beams are focusedinto the spectrometer with high spectral resolution by the secondconcave reflection mirror. Thus, the interference spectrum is obtainedfor the SRSI measurement.

The holes on the plate with four equal-sized holes may be of any shape.For example, the plate with the four equal-sized holes have the fourholes in a square shape.

The present invention has the following features:

(a) The transient-grating effect (the laser to be measured is dividedinto four beams here) is used as the generation of the reference lightin the measurement, where three of the four divided beams are used togenerate the reference light. It may run in the spectral range of200-3000 nm but the range is not so limited. Pulses with nanojoule levelfrom the oscillator can also be measured with the method of the presentinvention.

(b) The apparatus according to the present invention is very simple. Byusing a few mirrors, two glass plates, the interference spectrum betweenthe transient-grating signal and the pulse to be measured is obtained.

(c) In the present invention, the spectral interferometry is used toretrieve the spectral phase of the pulse. In the SRSI method, computersoftware programs may be used for the linear calculation, which issimple and fast. Only three-time iterative calculation is needed toobtain the spectral phase, pulse shape, and the temporal phase of thepulse.

(d) In comparison with the current technology, the present inventionsignificantly extends the applicable range of the femtosecond pulse tobe measured including the spectral range and pulse width. Themeasurement is fast and can be used as the single-shot measurement andfor real-time monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of the typical optical setup according tothe present invention.

FIG. 2 shows another embodiment of the typical optical setup accordingto the present invention.

FIG. 3 shows yet another embodiment of the typical optical setupaccording to the present invention.

FIG. 4( a) is a schematic view of the plate with four equal-sized holesused in the present invention, with the light is shown to be passingthrough the white part of the plate. L is the distance between thecenter of the two neighbor holes and d is the diameter of the whitehole.

FIG. 4( b) is a schematic view of the plane reflective mirror that thefirst, second, and third quadrants of the mirror are coated with highreflective film, and the fourth quadrant is uncoated; the white area isuncoated, the shaded area is coated.

FIG. 5 shows the phase retrieval processes of the SRSI method for thepresent invention.

FIGS. 6( a) and 6(b) show the data measured based on the setup of theoptical devices in FIG. 1, where the pulse used has the centerwavelength of 800 nm and pulse duration of 40 fs:

FIG. 6( a) shows the interference spectrum (thin-solid line) when thetime delay between the laser pulse to be test and the reference light is0.8 ps, the spectrum of the reference light (thick-solid line), and thespectrum of the laser pulse to be test (dotted line);

FIG. 6( b) shows the measured spectrum (solid line) and retrievedspectral phase (dotted line) of the pulse to be measured.

DETAILED DESCRIPTION OF THE INVENTION

Several embodiments of the present invention, together with thedescription are shown as follows, which serve to explain the principlesof the invention. The drawings are only for the purpose of illustrateseveral embodiments of the invention and are not to be construed aslimiting the invention.

The present invention uses a transient-grating light as a reference beamfor the SRSI measurement. The transient-grating light is generated basedon the transient-grating effect by using three beams overlapped on atransparent medium.

First, the transient-grating effect on a transparent dielectric materialis used to generate a reference light. The first embodiment of theapparatus and optical setup of the present invention is shown in FIG. 1.The optical setup includes an incident laser beam 1; an iris plate 2; aglass plate 3 which is used to introduce a suitable time delay; a planereflective mirror 4, of which the first, second, and third quadrants ofthe plane reflective mirror 4 are coated with high reflective film forlight tuning; a first concave mirror coated with high reflective film 5;a transparent medium 6 for generation of the transient-grating light; aniris 7 which is used to select the signal light and block stray light; asecond concave mirror with a reflective film 8; a spectrometer with highspectral resolution 9 for the measurement of the spectrum and thespectral interferometry.

In the optical setup in FIG. 1, plate 2 has four holes withequal-diameter which are arranged in a square shape. The incident laseris divided into four beams with equal diameter by the plate 2 as shownin FIG. 4( a). The plane reflective mirror 4 is shown in FIG. 4( b), ofwhich the first, second, and third quadrant are coated, and the fourthquadrant is not coated.

In FIG. 1, beam 1 is large enough (for example, larger than 5 mm) tocover the plate 2. After passing though the plate 2, beam 1 is dividedinto four beams with equal beam diameter. The four laser beams locatedon the four corners of a square, formed a so-called “box shape” (box).

One of the four beams passes though a glass plate 3 with suitablethickness. The other three laser beams of the four beams propagate inthe free air. Then, there is suitable time delay between the beam thatpasses through the glass plate 3 and the other three beams. Then, thefour beams are reflected by the plane reflective mirror 4. The beam thatis time delayed is reflected by the non-coated quadrant of mirror 4. Theother three beams are reflected by the three coated quadrants of mirror4, respectively.

After reflecting by mirror 4, the four beams are reflected onto thefirst concave reflective mirror 5 with a small incident angle. Then, thefour beams are focused onto the glass medium 6 after mirror 5. The threebeams reflected by the coated parts are overlapped on the dielectricglass 6 both in time and space. The transient-grating light isgenerated, which is on the direction of the time delayed beam and isoverlapped with it in space. By using the iris 7, the transient-gratinglight and the beam to be measured (the beam with suitable time delay)are selected. After focusing by using a second concave reflective mirror8, the spectral interferometry is measured by the spectrometer 9 withhigh spectral resolution.

In the device of the present invention, the diameter and distance of thefour holes on the plate 2 are chosen by the incident beam. The design isbased on the principles that the four beams will not affect each other.The glass plate 3 is selected according to the laser wavelength whichshould be transparent for the glass and the dispersion is small. Thethickness of the glass plate 3 should be thin if the spectral bandwidthis broad and the pulse duration is short. It will be limited by thespectral resolution of the spectrometer 9. Based on the wavelength ofthe incident pulse, the plane reflective mirror 4 and the first concavereflective mirror 5 can be coated with silver, gold, aluminum, or a highreflective dielectric film. The glass medium 6 is transparent to theincident laser pulse, and preferably, has a relatively high third-ordernonlinear coefficient. The thickness of glass plate 6 is usuallyselected to be 100-500 um. Preferably, the spectrometer 9 has a highspectral resolution.

In principle, the transient-grating effect is described by theexpression (1):

$\begin{matrix}{{I_{TG}\left( \omega_{TG} \right)} \propto {\begin{matrix}{\int{\int{{\omega_{1}}{\omega_{2}}\chi^{(3)}{{\overset{\sim}{E}}_{1}^{*}\left( {z,\omega_{1}} \right)}{{\overset{\sim}{E}}_{2}\left( {z,\omega_{2}} \right)}}}} \\{{{\overset{\sim}{E}}_{3}\left( {z,{\omega_{TG} - \omega_{2} + \omega_{1}}} \right)}\sin \; {c\left( {\Delta \; {k_{z}\left( {\omega_{TG},\omega_{1},\omega_{2}} \right)}{L/2}} \right)}}\end{matrix}}^{2}} & (1)\end{matrix}$

where ω_(TG), ω₁ and ω₂ are the transient-grating light, two incidentlights, respectively. Δk_(z)(ω_(TG), ω₁, ω₂) is the phase mismatch, L isthickness of the nonlinear dielectric material.

According to the expression (1), the generated transient-grating lightown a smoother and wider spectrum than that of the incident laser pulse.As a result, the generated transient-grating light is used as thereference pulse for the SRSI measurement.

In the SRSI measurement, the generated transient-grating light (namedreference light hereafter) together with the time delayed pulse to bemeasured are focused into a spectrometer with high spectral resolution.

The laser pulse to be measured is blocked at first to measure thespectrum of the reference light. Then, the other three beams is shieldedso that no reference light is generated. The spectrum of the laser pulseto be measured is obtained by the spectrometer. By adjusting the pulseenergy of the incident laser, the ratio between the reference light andthe pulse to be measured is adjusted to suitable value (for example, thereference light is about 3 times stronger than the that of the pulse tobe measured). Then, the spectral interference is measured and the datais saved.

By changing the thickness of the glass plate 3, the time delay betweenthe reference light and the pulse to be measured can be tuned. Clearinterference fringes can appear at suitable time delay τ. Theinterference fringes increase with the increase of the time delay. Itcan increase the accuracy of the measurement of the spectrum andspectral phase, but it also requires a spectrometer with a higherspectral resolution. In the example, the time delay τ is adjusted tomake the spectral fringes interval width at about 2 nm. The two laserbeams are optimized to get the maximum modulation depth spectralinterference fringes D(ω,τ) and save the data. The measured spectralinterference fringes D(ω,τ) can be expressed as:

$\begin{matrix}\begin{matrix}{{D\left( {\omega,\tau} \right)} = {{{E_{ref}(\omega)} + {{E(\omega)}^{{\omega}\; \tau}}}}^{2}} \\{= {{{E_{ref}(\omega)}}^{2} + {{E(\omega)}}^{2} + {{f(\omega)}^{{\omega}\; \tau}} + {{f^{*}(\omega)}^{{- }\; \omega \; \tau}}}}\end{matrix} & (2)\end{matrix}$

where ω is the angular frequency of the laser,S₀(ω)=|E_(ref)(ω)|²+|E(ω)|² is the sum spectrum of the reference pulseand the pulse to be measured; ƒ(ω)=E*_(ref)(ω)E(ω) is the interferenceterm of the two laser beams.

Subsequently, the spectrum and spectral phase of the pulse to bemeasured can be retrieved by using the SRSI method, and then obtain thepulse width and shape.

The calculation process of the SRSI method is shown as follows:

The initial spectral phase is set to 0, the spectrum and spectral phaseof the pulse to be measured may be calculated by using Fouriertransformation and iterative procedure shown in FIG. 5, where S₀(τ),ƒ(τ) are the Fourier transformation of the S₀(ω) and ƒ(ω) in the timedomain, respectively. To obtain the laser spectrum and spectral phase,it needs the following steps as shown in FIG. 5:

1. Fourier transform the measured interference spectrum D(ω,τ) into thetime-domain signal;

2. Extracted the time domain signals S₀(τ), ƒ(τ) out by using a windowfunction (such as super-Gaussian function);

3. Inverse Fourier transform S₀(τ) and ƒ(τ) to the frequency domain, andobtain S₀(ω) and ƒ(ω), respectively;

4. By using the following linear formulas together with S₀(ω) and ƒ(ω),we can obtain the spectral amplitudes of both the pulse to be measuredand the reference light, which are |E(ω)| and |E_(ref)(ω)|,respectively:

|E _(ref)(ω)|=½·(√{square root over ((S ₀(ω)+2|ƒ(ω)|))}{square root over((S ₀(ω)+2|ƒ(ω)|))}+√{square root over ((S ₀(ω)−2|ƒ(ω)|))}{square rootover ((S ₀(ω)−2|ƒ(ω)|))})  (3)

And

|E(ω)|=½·(√{square root over ((S ₀(ω)+2|ƒ(ω)|))}{square root over ((S₀(ω)+2|ƒ(ω)|))}−√{square root over (S ₀(ω)−2|ƒ(ω)|))}{square root over(S ₀(ω)−2|ƒ(ω)|))})  (4)

As a result, we can obtain the laser spectra of the pulse to be measuredand reference pulse, which are |E(ω)|² and |E_(ref)(ω)|², respectively.

5. After unwrapping the ƒ(ω), the spectral phase of the pulse to bemeasured can be calculated iteratively by using the following formula:

φ(ω)=φ_(ref)(ω)+arg ƒ(ω)+C  (5)

where, φ(ω) and φ_(ref)(ω) are the spectral phases of the pulse to bemeasured and that of the reference light (initial phase is assumed to be0), C is the phase constant induced by the dispersive optical elements;

6. The obtained laser spectrum and spectral phase are Fouriertransformed to time domain. Then, the pulse shape |E(t)|² and the pulsewidth of the pulse to be measured are obtained, while E(t) is theFourier transform value of E(ω);

7. Because the spectral phase of the reference light is not exactlyequal to zero, it needs a further iterative calculation step to obtainoptimized laser spectrum and spectral phase. The iterative calculationsare shown as follows:

(i) Based on the result obtained in the step 6, the shape of theelectric field of the reference light can be expressed as E(t)*|E(t)|²according to the formula (1). Through Inverse Fourier transformation ofE(t)*|E(t)|², the spectrum and spectral phase of the reference light areobtained, which are |E_(ref)(ω)|² and φ_(ref)(ω), respectively.

(ii) Based on the spectral phase of the reference light φ_(ref)(ω)obtained in the above step (i) and the formula φ(ω)=φ_(ref)(ω)+argƒ(ω)+C, an optimized spectral phase of the pulse to be measured can beobtained. After Fourier transformation of the new spectrum and newspectral phase of the pulse to be measured, the pulse shape and pulsewidth of the pulse to be measured are obtained;

(iii) After repeating above steps (i) and (ii) by several times, theoptimized spectrum and spectral phase of the pulse to be measured areobtained. Finally, the corrected laser spectrum, pulse shape, and pulseduration of the pulse to be measured are obtained.

Example

An apparatus that uses the optical setup of FIG. 1 is used, afemtosecond pulse from a commercial laser system (Coherent Inc.) ismeasured. The incident laser 1 to be measured have a repetition rate of1 kHz, a center wavelength of 800 nm, a beam diameter of 15 mm, and apulse energy of 10 μJ. The incident beam 1 is divided into four beamsafter passing through the plate 2. The beam on the right-lower corner ofthe four beams passes though a glass plate 3 with 0.5 mm thickness. Theother three laser beams of the four beams propagate in the free air.Then, the four beams are reflected by the plane reflective mirror 4. Thebeam that passes through the 0.5 mm thickness glass plate is reflectedby the non-coated quadrant of mirror 4. The other three beams arereflected by the coated first, second, and third quadrants of mirror 4,respectively. After being reflected by mirror 4, the four beams arereflected onto the first concave reflective mirror 5 with a radius of600 mm. Then, the four beams are focused onto a CaF₂ plate 6 with 150 μmthickness. The three beams reflected by the coated parts are overlappedon the CaF₂ both in time and space. The transient-grating light isgenerated, which is on the direction of the time delayed beam and isoverlapped with it in space. By using an iris 7, the transient-gratinglight and the beam to be measured (the beam with suitable time delay)are selected. After focusing by using a second concave reflective mirror8, the spectral interferometry is measured by the spectrometer 9 withhigh spectral resolution.

FIG. 6( a) shows the interference spectrum (thin-solid line) when thetime delay between the laser pulse to be test and the reference light is0.8 ps, the spectrum of the reference light (thick-solid line), and thespectrum of the laser pulse to be test (dotted line). Based on themeasured interference spectrum, the spectrum and spectral phase of thepulse to be measured are obtained by using the calculation process shownin FIG. 5. FIG. 6( b) shows the measured spectrum (solid line) andretrieved spectral phase (dotted line) of the pulse to be measured.

As a result of using the method of the present invention, the pulsewidth and shape are obtained. In the method of the present invention,only two or three reflective mirrors are used. The setup is very simpleand does not need polarizer that will induce dispersion to the measuredpulse. As a result, the method can be used to measure ultrashort pulsein the range of 10-300 fs at different wavelength. It can also be run insingle-shot or be used for real-time monitoring of femtosecond laserpulse. Then, the spectral phase measured can be fed back to phasecompensative device and optimize the femtosecond laser pulse.

We claim:
 1. An apparatus for measuring femtosecond laser pulse,comprising a plate having four equal-sized holes and being positioned onan optical pathway to receive incoming laser to be measured, a planereflective mirror being positioned on the optical pathway behind theplate and having a first, a second, a third, and a fourth quadrants, adelay plate being positioned on the optical pathway behind one of thefour equal-sized holes of the plate and between the hole and the fourthquadrant of the plane reflective mirror, a first concave reflectivemirror being positioned on an optical pathway of reflected beams by theplane reflective mirror and having a focal point, a third-ordernonlinear optical medium being positioned on the focal point of thefirst concave reflective mirror, an iris being positioned on an opticalpathway of beams after passing through the third-order nonlinear opticalmedium, a second concave reflective mirror being positioned behind theiris on the optical pathway of beams, and a spectrometer beingpositioned to received beams after being focused by the second concavereflective mirror and having high spectral resolution, wherein the platereceives the incoming laser and divides the laser into a first, second,third, and fourth beams through the four equal-sized holes; the first,second, and third quadrants of the plane reflective mirror directlyreflect the first, second, and third beams, respectively; the delayplate is positioned to have the fourth beam passing therethrough, andthe fourth beam is then reflected by the fourth quadrant of the planereflective mirror; the first concave mirror is positioned to reflect thefirst, second, third, and fourth beams; the third-order nonlinearoptical medium is positioned where the first, second, and third beamsare overlapped therein to generate a transient-grating signal light thatcollinearly overlaps with the fourth beam in space; the iris ispositioned to have the transient-grating signal light and the fourthbeam passing through; the second concave reflective mirror is positionedbehind the iris and focuses the transient-grating signal light and thefourth beam into the spectrometer, the spectrometer is positioned toreceive the transient-grating signal light and the fourth beam afterbeing focused by the second concave reflective mirror, whereby aninterference spectrum is obtained for SRSI measurement.
 2. The apparatusfor measuring femtosecond laser pulse according to claim 1, wherein thefirst, second, and third quadrants of the plane reflective mirror arecoated with high reflective film, and the fourth quadrant of the planereflective mirror is uncoated.
 3. The apparatus for measuringfemtosecond laser pulse according to claim 1, wherein each of the fourequal-sized holes of the plate is of a square shape,
 4. The apparatusfor measuring femtosecond laser pulse according to claim 1, wherein thedelay plate is a neutral density filter.
 5. An apparatus for measuringfemtosecond laser pulse, comprising a plate having four equal-sizedholes and being positioned on an optical pathway to receive an incominglaser, a delay plate being positioned in the optical pathway behind oneof the four equal-sized holes of the plate, a lens being positioned inthe optical pathway behind the plate with the four equal-sized holes andthe delay plate, the lens having a focal plane, a plane reflectivemirror being positioned behind the lens and having a first, a second, athird, and a fourth quadrants, the first, second, and third quadrants ofthe plane reflective mirror being coated with high reflective film andthe fourth quadrant being uncoated, a third-order nonlinear opticalmedium being positioned on an optical pathway of beams after beingreflected by the plane reflective mirror and being located at the focalplane of the lens, an iris being positioned on the optical pathway ofthe reflected beams after passing through the third-order nonlinearoptical medium, a concave reflective mirror being positioned behind theiris on the optical pathway of the reflected beams, and a spectrometerbeing positioned to received beams after being focused by the concavereflective mirror and having high spectral resolution, wherein the platewith the four equal-sized holes divides the laser into a first, second,third, and fourth beams through the four equal-sized holes; the lens ispositioned between the plate with the four equal-sized holes and theplane reflective mirror whereby the first, second, and third beamsdirectly pass through the lens; the delay plate is positioned betweenone of the four equal-sized holes of the plate and the lens whereby thefourth beam passes the delay plate and then the lens; the first, second,and third quadrants of the plane reflective mirror are positioned todirectly reflect the first, second, and third beams, respectively, andthe fourth quadrant of the plane reflective mirror is positioned toreflect the fourth beam; the third-order nonlinear optical mediumlocated at the focal plane of the lens is positioned where the first,second, and third beams are overlapped therein to generate atransient-grating signal light that collinearly overlaps with the fourthbeam in space; the iris is positioned to have the transient-gratingsignal light and the fourth beam passing through; the concave reflectivemirror is positioned to focus the transient-grating signal light and thefourth beam into the spectrometer, the spectrometer is positioned toreceive the transient-grating signal light and the fourth beam afterbeing focused by the concave reflective mirror, whereby an interferencespectrum is obtained for SRSI measurement.
 6. The apparatus formeasuring femtosecond laser pulse according to claim 5, wherein each ofthe four equal-sized holes is of a square shape.
 7. The apparatus formeasuring femtosecond laser pulse according to claim 5, wherein thedelay plate is a neutral density filter.
 8. An apparatus for measuringfemtosecond laser pulse, comprising a plate having four equal-sizedholes and being positioned on an optical pathway to receive incominglaser to be measured, a first concave reflective mirror being positionedon the optical pathway behind the plate and having a focal point and afirst, a second, a third, and a fourth quadrants, the first, second, andthird quadrants of the plane reflective mirror being coated with highreflective film and the fourth quadrant being uncoated, a delay platebeing positioned on the optical pathway behind one of the fourequal-sized holes of the plate and between the hole and the fourthquadrant of the first concave reflective mirror, a third-order nonlinearoptical medium being positioned on the focal point of the first concavereflective mirror and on an optical pathway of beams after beingreflected by the first concave reflective mirror, an iris beingpositioned on the optical pathway of the reflected beams after passingthrough the third-order nonlinear optical medium, a second concavereflective mirror being positioned behind the iris on the opticalpathway of the reflected beams, and a spectrometer being positioned toreceived beams after being focused by the second concave reflectivemirror and having high spectral resolution, wherein the plate receivesthe incoming laser and divides the laser into a first, second, third,and fourth beams through the four equal-sized holes; the first, second,and third quadrants of the first concave reflective mirror directlyreflect the first, second, and third beams, respectively; the delayplate is positioned to have the fourth beam passing therethrough, andthe fourth beam is then reflected by the fourth quadrant of the firstconcave reflective mirror; the third-order nonlinear optical medium ispositioned where the first, second, and third beams are overlappedtherein to generate a transient-grating signal light that collinearlyoverlaps with the fourth beam in space; the iris is positioned to havethe transient-grating signal light and the fourth beam passing through;the second concave reflective mirror is positioned to focus thetransient-grating signal light and the fourth beam into thespectrometer, the spectrometer is positioned to receive thetransient-grating signal light and the fourth beam after being focusedby the second concave reflective mirror, whereby an interferencespectrum is obtained for SRSI measurement.
 9. The apparatus formeasuring femtosecond laser pulse according to claim 8, wherein each ofthe four equal-sized holes is of a square shape.
 10. The apparatus formeasuring femtosecond laser pulse according to claim 8, wherein thedelay plate is a neutral density filter.
 11. The apparatus for measuringfemtosecond laser pulse according to claim 8, wherein the delay plate isa transparent glass, the concave mirror is a concave reflective mirror,the first, second, and third quadrants of the concave mirror are coatedwith high reflective film, and the fourth quadrant of the concave mirroris uncoated.
 12. A method for measuring femtosecond laser pulse based ontransient-grating effect according to claim 1, comprising generating areference light for a transient-grating effect in the third-ordernonlinear medium, obtaining a spectral interference fringes D (ω, τ)between femtosecond pulses to be measured and the reference light by thespectrometer with high spectral resolution, and retrieving a laserspectrum and spectral phase of the femtosecond pulses to be measuredbased on the spectral interference fringes D (ω, τ) by SRSI inversioncalculation to obtain pulse width and pulse shape of the laser.
 13. Amethod for measuring femtosecond laser pulse based on transient-gratingeffect according to claim 5, comprising generating a reference light fora transient-grating effect in the third-order nonlinear medium,obtaining a spectral interference fringes D (ω, τ) between femtosecondpulses to be measured and the reference light by the spectrometer withhigh spectral resolution, and retrieving a laser spectrum and spectralphase of the femtosecond pulses to be measured based on the spectralinterference fringes D (ω, τ) by SRSI inversion calculation to obtainpulse width and pulse shape of the laser.
 14. A method for measuringfemtosecond laser pulse based on transient-grating effect according toclaim 8, comprising generating a reference light for a transient-gratingeffect in the third-order nonlinear medium, obtaining a spectralinterference fringes D (ω, τ) between femtosecond pulses to be measuredand the reference light by the spectrometer with high spectralresolution, and retrieving a laser spectrum and spectral phase of thefemtosecond pulses to be measured based on the spectral interferencefringes D (ω, τ) by SRSI inversion calculation to obtain pulse width andpulse shape of the laser.